How to resolve error when accepting update for appointment? ("This meeting is not in the Calendar; it may have been moved or deleted.")

When an executive is invited to an appointment, The invitation is in his Inbox
and the appointment puts itself in his calendar like it does for
everyone else.
When the invitation is updated he gets a message "This
meeting is not in the Calendar; it may have been moved or deleted." If
he accepts the message, his calendar is updated. This is reproducible
for executive but not on anyone else here.

1st level support has  tried all the standard troubleshooting. Outlook /Cleanfreebusy
/cleanreminders, Deleted forms cache, new profile. Nothing fixed it.

FYI - Being an Executive, he has a larger mailbox size.

Thanks